PM: BWOD re aromatisation

In chemistry, a molecule is called aromatic not if it smells nice but if it has atoms in a ring.

You can think of molecules as made up of atoms, each atom having a number of sticky tabs. Hydrogen has one tab, Oxygen has two and Carbon has four.

So if you think about water, which we all know is H2O, or two hydrogens and one oxygen, it should be fairly clear that the oxygen atom sits "in the middle" and the two hydrogens each stick out (actually at an angle, but don't worry about that now) something like H-O-H - as you see each hydrogen is using its one tab and each oxygen is using both of its tabs.

If you take a string of six carbon atoms all in a row with hydrogens on the "spare" tabs, you get something like this

C-C-C-C-C-C

Each end carbon will have three hydrogens and all the middle carbons will have two.

Now imagine taking that molecule (which is called hexane), knocking a hydrogen off each of the end carbons and sticking them together. You've got an aromatic molecule. What actually happens is a little bit more complex, you lose some more hydrogens and you get a chain like

(joined to the sixth one)-C=C-C=C-C=C-(joined to the first one)

in a hexagonal ring, each carbon uses three of its sticky tabs, two to stick to another carbon on one side and one to stick to another carbon on the other side; and the fourth sticky tab is used to stick to a Hydrogen. What you have here is benzene, or C6H6, the first known and studied aromatic molecule (the actual structure is a bit more complex than I'm making out here, it's more like each carbon uses one and a half of its tabs on each side with its neighbouring carbons, but that makes the tabs analogy break so I'm pretending it doesn't happen that way). The key thing is that each carbon in the ring uses three of its tabs to contribute to the ring bonding.

Carbon rings with six carbons are the most common, but you can get carbon rings with five carbons too and sometimes a ring with four or five carbons and one, two or three nitrogens (nitrogen has three sticky tabs to play with) or sometimes other atoms. You can get hexagons and pentagons which sit next to each other with two atoms being a side of both the hexagon and the pentagon. For an example molecule with quite a lot of this kind of thing look up the structure of codeine on Google.

Not all ringed molecules are aromatic. Testosterone, as a steroid, has four concurrent rings - but one of them is not an aromatic ring (it has a double bond out to an oxygen - so one of the carbons only has two tabs which it uses to bind to the carbons either side of it, because its other two are both bound to an oxygen, using both of that oxygen's tabs). But to be aromatic, each atom in the ring must use three of its tabs to make up the ring bonds.

Aromatase breaks that oxygen double bond and sticks a hydrogen on the other side of the oxygen, so the carbon now has its three tabs to contribute to the ring and the ring is now aromatic; the oxygen has one tab to the remaining tab of that carbon and its other is now to a hydrogen (again I'm skating over a detail or two - a methyl group also gets knocked off). This is called aromatisation.

Aromatic molecules are typically stable i.e. not very reactive, because this way of sharing tabs around a ring is hard to break up. They're called aromatic because the first studied group were derivatives of benzene (C6H6 as above) and many of those compound do indeed smell, more or less pleasant.

Posted By: Old Man, Jul 26, 13:40:11